Conventional passive sonar systems detect acoustic signals emanating from an “underwater object;” that is, any device that moves through the water while emitting acoustic signals that sonar can detect. Torpedoes and submarines are examples of such “underwater objects.” Sonar systems employ a variety of technologies to detect and locate such underwater objects. Generally, however, acoustic signals from a specific object define a complex wavefront and pass a towed horizontal array or a stationary vertical array of hydrophones at different times. U.S. Pat. No. 4,965,732 to Roy et al. for Methods and Arrangements for Signal Reception and Parameter Estimation and U.S. Pat. No. 5,216,640 to the same inventors as in this application for an Inverse Beamforming Sonar System and Method are examples of such sonar systems. Some sonar systems, implement a beamforming process to correlate the signals received at each hydrophone and to decompose these signals into a signal representing the bearing to a particular underwater object.
Generally, for a linear array of hydrophones towed behind a vessel such as a surface ship or submarine, the longer and straighter that linear array is, the better one can focus those elements of the array to “look” in a particular direction. A problem arises, however, during ship maneuvers wherein a course change causes a distortion of the towed array. If one does not know the array shape while the array is distorted, target tracking and focus is impaired.
Prior compensation techniques for the aformentioned problem include estimating the array shape based on the vessel's own change in course and resulting path through the water, and providing heading sensors on each of the hydrophone array elements, for example. Models that estimate array shape based on course maneuvers have proved problematic to implement and in certain cases invalid (e.g. for long arrays of about 400 hydrophones, or more) as empirical data tends to show that a towed array does not follow the channel formed by the vessel during own ship maneuvers. Moreover, during severe maneuvers, heading sensors disposed on the hydrophones are extremely inaccurate and hence do not provide adequate information to accurately track contacts during such periods. In tactical situations, such as tracking a potentially hostile target, a relatively severe maneuver may be necessary to evade an action by the target (e.g. a torpedo fire). It is most desirable to maintain the track of the target through the maneuver.
Another technique, commonly called the “bright star technique”, utilizes a known strong signal emanating, for example, from a noisy surface ship. In this method, a large number of possible array shapes are processed and the signal data is beamformed based on the input array shapes. The strongest response in the direction of the strong signal is used to focus the beam in that direction. This approach, however, requires both a strong signal and a relatively long time interval for focusing and adjusting the input array parameters to arrive at a focused output beam. In many tactical situations, however, neither a strong target nor a long conversion time is available. Accordingly, it is highly desirable to obtain a system and method for quickly and efficiently maintaining the track of a target when the array shape is unknown and/or when heading sensor data is unreliable or unavailable.